feat(market): embed offer photos inline instead of a media server

Replace the Blossom media-server approach with a fully decentralized one:
generate a small (~320px) JPEG thumbnail from the lot's cover photo and
embed it in the offer event as a data: URI, so photos ride the relays with
no server and no IP leak. The full photo stays in the encrypted inventory.

- offer_thumbnail.dart: downscale-until-it-fits data-URI builder + decoder
- market_widgets: render data: URIs from memory, http(s) URLs from network
- offers_cubit: publish a thumbnail (best-effort) in place of an upload
- drop MediaTransport/Blossom from commons_core (http/crypto deps) and the
  media-server setting; parked on branch parked/offer-photos-blossom

Relay event-size limits cap the image to a thumbnail; higher-res sharing
would need the parked Blossom path.

apps/app_seeds/lib/services/social_settings.dart

@@ -17,7 +17,6 @@ class SocialSettings {
   static const _areaKey = 'tane.social.area_geohash';
   static const _relaysKey = 'tane.social.relays';
   static const _searchPrecisionKey = 'tane.social.search_precision';
-  static const _mediaServerKey = 'tane.social.media_server';
 
   /// How wide "your zone" searches, as a geohash prefix length: 5 ≈ ±2.4 km
   /// ("very close"), 4 ≈ ±20 km ("around here"), 3 ≈ ±78 km ("my region").
@@ -40,11 +39,6 @@ class SocialSettings {
     'wss://relay.primal.net',
   ];
 
-  /// Blossom media server used to host offer photos so peers can see them. The
-  /// Comunes-run server is the non-commercial default; the user can point this
-  /// at any Blossom server, or clear it to publish offers without photos.
-  static const String defaultMediaServer = 'https://blossom.comunes.org';
-
   /// The user's coarse area as a low-precision geohash (may be empty). Set
   /// manually or from device location; the transport coarsens it further.
   Future<String> areaGeohash() async => (await _store.read(_areaKey)) ?? '';
@@ -79,18 +73,6 @@ class SocialSettings {
   Future<void> setSearchPrecision(int precision) =>
       _store.write(_searchPrecisionKey, '${_clampPrecision(precision)}');
 
-  /// The Blossom media server for offer photos. Falls back to
-  /// [defaultMediaServer] until the user configures one; an explicitly saved
-  /// empty value turns photo hosting off (offers still publish, without images).
-  Future<String> mediaServerUrl() async {
-    final raw = await _store.read(_mediaServerKey);
-    if (raw == null) return defaultMediaServer; // never configured
-    return raw.trim();
-  }
-
-  Future<void> setMediaServerUrl(String url) =>
-      _store.write(_mediaServerKey, url.trim());
-
   int _clampPrecision(int p) => p < minSearchPrecision
       ? minSearchPrecision
       : (p > maxSearchPrecision ? maxSearchPrecision : p);